Automatic volume regulator



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AUTOMATIC VOLUME REGULATOR Cornelis Bevoort-and .Raimond Edouard MarieBaudet, TheHague, Netherlandgrassignors to Staatsbedrijf-der Posterijen,Telegraiieen ,Telefonie, The Hague, Netherlands Original application May26, .1951,=Serial No. 228,394. Divided and this'application-March 18,1957, Serial No. 649,270

Claims priority, appiication Netherlands June 16, 1950 9.Claims.(Cl.:179--1) The invention relates to a system for automatic control ofthe speech current'level, e; g. at the output of a radioreceiver.

A device of this kind has'beendescribed in the application which Wasfiled on May 26, 1951, by Bevoort et al. and assigned Serial No.228,394, and of which the present application is a division. This typeequipment was also disclosed in the U. S. A. patent application No.189,412.

A characteristic of this system is the simple manner in which the outputlevel can be regulated. A transmission measuring set, usually called TMSconnected to the output controls minimum and maximum relays, contacts ofwhich switch in a motor for adjusting an adjustable attenuator at theinput of the apparatus.

The system of the invention intends to procure an automatic volumeregulator which is in a high degree independent of the occurrence ofnoise, whistling, etc. This result has been secured by means of a devicewhich determines the presence of speech of usable level whichparticularly responds to telephone currents occurring in the mixture ofalternating currents applied to the input.

This device, which will be called discriminator hereinafter, consists ofa preamplifier, a first rectifier, an amplifier, a second rectifier anda relay; in this arrangement the rectification and smoothing of thespeech currents obtained from the preamplifier yield a voltage varyingin a syllabic rhythm, which voltage, after amplification once morerectified, controls, accordingly as there are telephone currents presentat the input or not, a relay inserted inthe anode circuit of theterminal tube, the action of the said relay being delayed by appropriatemeans, sothat between the syllables the armature is not actuated-whereasby the application of delayed automatic volume controlin the saidpreamplifier the switching times are independent of the signal level.

Another characteristic of the invention is such an arrangement that inthe absence of speech of usable level the.;discriminator.switches 011?the motor regulator before the rninimumrelay of the TMS has released,and that when telephone currents arrive again, the latter relay closesits contact .again before the discriminator relay -As a result anundesired start of the motor, perhaps 'in a wrong direction, is avoided.

According to the invention the discriminator is insensible to noisesofshort duration, as the discriminator relay is prevented'trom attractingits armature as long as the preamplifier has not yet found its correctadjustment. The rneanS by which this is achieved will be describedhereinafter.

According to a further characteristic of the invention .thediscriminator controls at the same time a noise limitatent rivesp-speechof usable level.

f iCQ "2 nexcd drawing, which shows, by way of example, someembodimentapviz:

Fig. 1 a. general view of the automatic volume regulator;

Fig. 2a an embodiment-of the discriminator;

Fig. 2b another embodiment of the discriminator;

Fig. 3 the transmission measuring set;

Fig. 4 a graph illustrating the operation of the TMS; Fig. 5 thearrangement .of the motor circuit of the regulator;

Fig. 6 the noise limiter.

Suppose the circuit]. to be connected to a radio-receiver. it isprolonged via an adjustable attenuator R, an amplifier Vs, a noiselimiter G0 and the circuit 2. The radio receiver is e. g. a receiver inacommercial communication and may be located in a receiving centre.Circuit 12 is the beginningof the incoming circuit (half of a four-wirecircuit) to a tour-wire termination set in the telephone exchange,whence a two-wire circuit leads to the subscribers premises, the otherhalf of the four-wire circuit leadingto a transmitting centre.

The discriminatorD, which is connected in parallel to the input, .willonly demagnetize relay S in case there var- In the chosen example thelimitsfor commercial input level at 1 lie between +6 and 34 db. Relay Sof the discriminator interrupts the motor circuit, so that forspeecharriving anew the same adjustment-is maintained.

The transmission measurement set NM connected via an attenuator V1 inparallel to the output of amplifier Vs determines, in the'absenceoispeech of usable level, the necessity of re-adjustment. The input levelof this device lies e. gm 5 db, so that by giving attenuator Vz asuitable attenuation value, -e. g. of ll db., the output level can bekept constant at 6 db. By re-adjusting the-attenuator -Vz any desiredvalue can be easily obtained.

in case the level is too high or too low the TMS will energize itsmaximum relay MA, or cause the release of its minimum relay Mi,respectively. In both cases the motor regulator -MR is switched in, butin opposite directions. In-the case of a too low level the motorregulator is not immediately switched in by relay Mi, but by theintermediary of a slowly releasing relay T. This is necessary in orderto give the discriminator the time to interrupt the motorcircuit in theabsence of speech of usable level,.in which case the regulator shouldnot operate. This relay T, however, should operate quickly in order toprevent the motor from being switched in when the discriminator relayreleases as a result of newly arriving speech.

Fig. 2a gives a diagram of the discriminator. A. C. voltages found incircuit 1 (Fig. 1) are applied .to the primary of transformer TR (points3 and 4). The

voltages developing across the secondary winding are applied between thegrids and cathodes of two tubes B1 and B2, whichare connected inparallel. As the discriminator should work at level of from +6 to 40 dbone amplifier tube proved insufficient. In order to obtain a certain AVCin this tube arrangement, the voltage variationswhich develop across theself-inductor in the anode circuit are vapplied via condenser C4 todiode B3. The

cathode. of the latter is connected to a tap on the potentiometerarrangement li -R and receives from this tap a constantvoltage which isthus chosen that in case the potential of point p rises above theno-signa1 value, the

diode is conductive, whereas in case the said potential ,falls thediodecarries no current.

The

i as for strong signals the mutual conductance is very low.

The voltage variations which develop across the choke coil L1 arerectified by diode B4. The audio frequency component of the rectifiedvoltage obtained across the points q and r is absorbed by theoscillatory circuit constituted by the parallel arrangement of chokecoil L2 2 and condenser C6. circuit lies between and 15 C./S.Consequently the The resonance frequency of this impedance is very highfor these frequencies, so that between grid and cathode of amplifiertube B5 a voltage 3 is applied, which varies in syllabic rhythm. Thevoltage variations developing across resistance R in the anode circuitof this tube are once rectified, via condenser C8 by a diode-rectifierG1. Condenser C7 constitutes a short circuit for audio frequencyoscillations which might occur in the anode circuit of tube B5. Thecircuit of rectifier G1 goes via resistance R9, which is taken up in thecathode lead of tube B4.

In consequence of the presence of speech at the input a negative voltagewith respect to the cathode is applied to the grid of tube B6. Thisvoltage should not vary in a syllabic rhythm, however, therefore therectified voltage is smoothed by means of condenser C9 connected inparallel to rectifier G1 and resistances R18 and R19. On arrival of asyllable series of usable level tube B6 is biassed to cut-ofi', so thatrelay S releases. This relay cannot operate in the intervals between twosyllables;

this is achieved by the action of the said condenser C9 1 and resistanceR18R19, this arrangement having a large time constant. As a result ofthe connection of a rectifier G2 in parallel to resistance R19 condenserC9 is quickly charged, but it is slowly discharged. The discriminator isintended to ascertain the presence of telephone currents of a certainminimum level. It is necessary, however, to equalize level variations asmuch as possible; the fact in that higher voltages at the points s and 1cause a greater delay in the arrangement of C9, R18 and R19,

this delay influencing in its turn the operation of relay 7 S. By meansof the AVC in the preamplifier tubes and the negative feedback in thecircuits of the rectifiers B4 and G1, the desired independence of theinput level is obtained. Relay S closes its contact s in the circuitofthe motor regulator so that the TMS can control the latter according asit establishes the necessity to do so. Another embodiment of thediscriminator according to the invention will be explained by means ofFig. 2b.

The signal obtained from the'radio receiver is applied to the primary Iof transformer TRl, primary 11 passing on the signal to the variableattenuator. The secondary is connected to the grids of a variableamplifier tube. As in Fig. 2a a variable u tube has been connected inparallel with a sharp cut-off tube, with a view to the large range oflevels the discriminator has to deal with. Via condenser C5 theamplified signal is applied to the diode D1. By means of half-waverectification condenser C4 is rapidly charged, the rectified voltagebeing applied with some delay via R8 and C1 to the grids of tubes V1 andV2. The charge of condenser C1 takes place rather quickly, though not soquickly that the speech fluctuations would be impaired, Whereas thedischarge of C1 via. R8 and R11 is so muchdelayed that in a normalconversation the amplifier retains between the words the amplificationit has been automatically adjusted to. In

order to obtain with a level of 40 db, as much amplification as possiblethe diode D1 receives a biassing voltage derived from resistorRS, sothat with low levels the regulating action is suspended.

In order to obtain the syllabic frequency, the amplified signal isapplied, via condenser C6 and resistor R13, the function of which willbe set forth later, to the diode D2. This diode is so connected, thatthe other half-cycles of the A. C. voltage are rectified, so thatcondenser C7 is tertisic, that frequencies of from 5 to 15 C./S. are

allowed to pass, whereas lower and higher frequencies meet with aconsiderable attenuation.

The A. C. voltage energizing from the second filter is applied to thegrid of the tube V4, which is arranged as a plate detector. This tube isso adjusted that the anode current only begins to flow, when the voltagehas attained a value corresponding to a speech fluctuation of about 10db, in order to prevent noise fluctuating in speech rhythm fromentailing a wrong re-adjusting command. A relay S is located in theanode circuit of tube V4; this relay, shunted with condenser C13 is slowin operating as well as in releasing. It is to be noted that, incontradistinction to the embodiment of Fig. 2a the arrangement is suchthat relay S operates in the case of speech arriving.

The action of the diodes D3 and D4 is as follows: During a period ofsilence condenser C1 will discharge via resistors R8 and R11, which willlead to a larger amplification. Parasitic noises arriving under theseconditions may be amplified too much during the adjustment of thepreamplifier, so that condenser C7 is charged by diode D2 to a too highvalue, which again results in a too high voltage at the grid ofamplifier tube V3. This opens the possibility for relay S to operate andgive a command. In order to prevent condenser C7 from receiving a toohigh charge, a diode D3 has been connected in parallel to diode D2 andcondenser C7. Diode D3 is given such a bias voltage from resistors R24and R25 that at normal working voltages it is non-conductive.

In consequence, no higher voltage can appear across the diode D2. thanis determined by D3. In order to enhance the effectiveness of thismeasure, resistor R13 has been connected in series to the two diodes,which results in a still better limiting of the voltage. The limitationof the voltage appearing across diode D2 also ensures that with speecharriving, the voltage fluctuations are reduced, when the regulatingamplifier has not yet reached its correct adjustment, so that relay AWill operate less rapidly.

A second protection against the operation of relay A on account ofdisturbances is achieved by means of diode D4. The voltage obtained viatransformer Tr2 from the preamplifier is rectified by diode D4, which isbiased by such a voltage that with the normal operation of the wholeapparatus no rectification takes place. Condenser C14 is charged by D4and is connected to the negative grid bias of tube V4 in such a mannerthat the grid of this tube is rendered more negative by the operation ofD4. In consequence tube V4, which is arranged as a plate detector,becomes less negative, so that the chance of operating of relay A i oncemore reduced.

If the period of speech is sufliciently long (e. g. 1 second or more),so that the preamplifier has the time to find its correct adjustment,the A. C. voltage on the anodes of V and V can no longer increase in adegree as to exceed the bias voltages of D and D The diodes D and D areput out of action, so that the discriminator gets the sensitivitynecessary to energize relay A.

The arrangement and the relays of the TMS are quick operating. As aresult in an interval of silence the minimum relay of the TMS willrelease sooner than the delayed action relay S, so that at the end of aninterval of speech a command is given for increasing the speech longthat relay S will release before relay T. In order to make the relayoperate as quick as possible a resistance R30 shunted by a condenser C15has been connected in series with the relay winding. j

The TMS consists of an amplifier which controls via a rectifier the tworelays Ma and Mi. Fig. 3 shows by wayof example an embodiment favorablefor the invention.

The input, points 5 and 6, is connected via an attenuator Vz (Fig. l) tothe output amplifier Vs. The voltages occurring at these points areapplied via transformer TR to grid and cathode of tube 131. The voltagevariations found in the anode circuit of this tube are applied by meansof resistive coupling to tube B2. A rectifier G is connected to thelatter by means of inductive coupling. In the circuit of rectifier Gthere is a resistance R13, via Which the cathode current of the firsttube flows. The resulting negative feedback makes the amplifier workwith a great constancy. Rectifier G controls the two relays Ma and Mi.C5 is a smoothing condenser. The windings of the two relays, as well asthe shunt resistance across Ma are so chosen, that in the operatingrange both the relays are excited by equal numbers of ampere turns.

With an average input level relay Mi is energized. It releases if thelevel falls in such a degree that readjustment becomes necessary.opened, so that relay T releases, applying voltage to terminal 1 of themotor panel.

. It is clear that such an arrangement of the TMS makes it possible tokeep the output level. of amplifier Vs within an interval of say 15 dbby switching in the motor when the minimum limit, or maximum limit,respectively is exceeded. Said limits can be changed by readjusting therelays. Fig. 4 shows graphically the coherence between the input levelof the TMS and the relay currents and indicates at the same time theadjusting limits for a dynamic of 10, 15 and 20 db. I

It will be seen from the detailed diagram (Fig. 5) of the motor panelthat voltage applied to point 1, or 2, causes the operation of relay MH,or ML, respectively. In the former casevoltage applied to point 1 as aresult of too low levelrelay 'MH is energized in the following circuit:earth, point 1, winding MH, contact m1 IIL-maximum contact max, point 7,60 v. As a result the motor is switched in in order to increase thelevel: earth, point 8 p contact mh armature winding M, contact mhcontact mh field winding RB, contact 7121 contact m1, armature windingM, contact mlz contact wil field winding EB, resistor R8, contact mlpoint 7, -60 v.

In order to avoid distortion, overload of amplifiers, etc., inconsequence of a too high level, the reducing of high levels should beeffected as quickly as possible. For this reason contact ml connects aresistor R8, in series with the field winding. As a result of theweakening of the field the motor will turn at a higher speed.

As a simultaneous excitation of the two relays MH and ML, which occur inthe case of rapid level changes, would result in a short-circuit of thebattery voltage, contacts have been applied by which the relays breakeach others circuit.

In case it turns out to be impossible to readjust the level sufficientlyin an upward or downward direction, the regulator will reach its finalposition. If the level is too low relay MH is switched out by shaftcontact max.,

In that casecontact mi is which at the same time switches in lamp V3 andalarm relay AR. If the level is too highthe shaft contact min.disconnects relay ML and switches in lamp V and alarm relay AR. RelayAR, which operates in both cases, switches in an alarm system.

By means of the noise limiter G0 (Fig. 1) it is possible to eliminatenoise from the connection during the intervals of silence by increasingby say 20 db the attenuation of an attenuator inserted in theconnection; this attenuator can with advantage be controlled by thediscriminator. This principle can be realized in many different Ways. Afavorable embodiment of the noise limiter is one in which a static relayopens or closes by means of a bridge circuit of resistors. Fig. 6 showssuch an arrangement. If contant s is in the position shown rectifiercells G3 and G4 receive a blocking voltage and rectifier cells Gland G2receive a passing voltage, which results in a large attenuation from TR2to TR3. The taps on resistor R9 are placed at such points that thepolarity is reversed when contact s is closed, so that then therectifier cells G3 and G4 get a passing voltage and rectifier cells G1and G2 get a blocking voltage, so that the attenuation becomes small.Contact s is a contact of the discriminator relay S. The control of thenoise limiter by the discriminator secures the result that in theintervals of silence the noise is considerably weaker, which highlybenefits the quality of the connection.

It is pointed that the application of ,the noise limiter differs fromthe way in which it was used in the U. S. A.

patent applications Nos. 189,411 and 189,412, also in connection withthe location of the automatic volume regulator in the incoming branch.

It may be noted, finally, that all the contacts in motor circuit,discriminator TMS and noise limiter are shunted by series connections ofa condenser and a resistor as spark quenches and in general to eliminatedisturbances.

While we have illustrated and described what We regard to be thepreferred embodiment of our invention, nevertheless it will beunderstood that such is merely exemplary and that numerous modificationsand rearrangements may be made therein without departing from theessence of the invention, We claim:

1. An automatic volume regulator consisting of an input and an outputcircuit, a first attenuator means for adjusting the output speech levelof said regulator to a given mean value, a second attenuator meansconnected on the output side of said regulator for normally inserting alarge value of attenuation in said output circuit to limit the linenoise thereover, and discriminator apparatus to condition said firstattenuator means for operation only with receipt of speech frequencieswhich are of a usable level at the input side of said regulator andwhich are of a syllabic rhythm, and to simultaneously control saidsecond attenuator to remove said attenuation from said output circuit.

2. An automatic volume regulator consisting of an input and an outputcircuit, a first attenuator means for adjusting the speech level of saidregulator to a given mean value, a second attenuator means connected onthe output side of said regulator for normally inserting a fixed largesaid first attenuator means in its adjustment of said output level tosaid mean value following enablement thereof by said discriminatormeans.

3. An automatic volume regulator consisting of an input and outputcircuit, a first attenuator means operable to introduce attenuation intosaid output circuit for adjusting the output speech level of saidregulator between two given values, a level measuring set connected tothe output side of said regulator for measuring the output level thereofincluding a maximum and a minimum relay which are connected to controlsaid first attenuator means in its adjustment of said output levelbetween said values, a second attenuator means for normally inserting alarge value of attenuation in said output circuit to limit the noisethereover, anddiscriminator apparatus operable to condition said firstattenuator means for operation only with receipt of speech frequenciesof a usable level at the input side of said regulator and tosimultaneously control said second attenuator to remove said attenuationfrom said output circuit.

4. A regulator as set forth in claim 3 which includes an electromotordrive for operating said first attenuator means in its adjustment, and acontrol circuit for said motor including a first energizing circuitcontrolled by said maximum relay for effecting increased attenuation asrequired and a second energizing circuit-controlled by said minimumrelay and said discriminator for effecting decreased attenuation asrequired.

5. An automatic volume regulator consisting of an input and outputcircuit, a first attenuator means for adjustiug the speech level overthe output circuit of said regulator between two predetermined values, asecond attenuator means for normally inserting a large value ofattenuation in said output circuit to limit the noise thereover, a levelmeasuring set connected to the output side of said regulator formeasuring the output level thereof including a maximum relay operativeto control adjustment of said first attenuator means in the event thatthe measured output exceeds apredetermined maximum value, and a minimumrelay which is operative to effect reduction of said first attenuatormeans in the event that said measured output level is below apredetermined minimum value, discriminator apparatus operable tocondition said first attenuator means for operation only with receipt ofspeech frequencies of a usable level at the input side of said regulatorand to simultaneously control said. second'attenuator to remove saidattenuation from said output circuit, and time delay means forintroducing a time delay in the control of said first attenuator meansby said' minimum relay, which delay is sufiicient to permit theoperation of said discriminator apparatus to disable said firstattenuator means inthe event that the speech tenuator means which iscompleted by said time delay means after the elapse ofa given periodresponsive to receipt of speech frequencies of a value less than saidpredetermined minimum.

7. An automatic volume regulator consisting of an input and outputcircuit, a first attenuator means for adjusting the speech level overthe output circuit of said regulator between given values, discriminatorapparatus operable to condition said first attenuator means foroperation only with receipt of speech frequencies of a usable level atthe input side of said regulator, a level measuring set for measuringthe output level of said regulator and for controlling said firstattenuator means in its adjustment of said output level whenever saidat-' tenuator means is conditioned for operation by said discriminatormeans, a second attenuator means for nor-l mally inserting a large valueof attenuationin said out put circuit to limit the noise thereovercomprising a' static relay, a bridge circuit of resistors forcontrolling the opening and closing of said static relay, and controlmeans operated by said discriminator for controlling said bridge circuitin the adjustment'of the attenuation in said output circuit. a

8. An arrangement as set forth in claim 7 in which said discriminatorapparatus includes a control relay connected to be normally operativeand to be released responsive to receipt of speech frequencies ofa'usable level at the input side of said regulator, a first pair ofcontacts on said relay for controlling said first attenuator means, anda second set of contact means for controlling said second attenuatormeans.

9. An arrangement as set forth in claim 7 in which said secondattenuator means comprises an input transformer, an output transformer;a bridge circuit of resistors, a first pairof rectifier cells, and asecond pair of rectifier cells connected between said input and outputtransformers; a control relay controlled by said discriminator to adjustthe direction of current flow through said rectifier and resistor bridgesystem and thereby alternatively introduce attenuation into or removeattenua- I tion from said arrangement.

No references cited.

